The present invention relates to a process for producing a polyester resin having increased molecular weight and improved processability by subjecting a polyester resin having relatively low molecular weight to a bond reaction using a small amount of a binder and a slight amount of a catalyst, and to a process for producing a polyester resin molded product. More particularly, the present invention relates to a process for producing a polyester resin having increased molecular weight and improved processability by restoring and improving a recovered or recycled polyester molded product with reduced molecular weight and physical properties, and a process for producing a polyester resin molded product having improved basic physical properties, mechanical characteristics, and the like.
Straight-chain aromatic saturated polyesters, such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene-2,6-naphthalene carboxylate (PEN), have excellent physical properties, and are widely used as fibers, films, plastics and the like. In the field of plastics, these kinds of polyesters are widely used as high performance resin materials for molded products for automobiles, machine parts, electric and electronic materials, building materials, containers, various industrial products and the like.
In recent years, with a view to saving resources and protecting the environment, the need to reutilize used plastic products recovered from factory production lines or the general consumer market has been recognized worldwide. Also, the recycling of used bottles, films and the like which are made of polyester has been positively progressing. However, such crystalline polyester has practical problems in that great decreases in molecular weight easily occur, and there is a strong tendency for the number of free carboxyl groups at the molecule terminals to increase due to the thermal history of mold working. This has hindered the development of reutilization techniques for recovered products. Since the molecular weight of recovered used polyester is decreased as compared with new chips, even flakes (crushed materials) derived from large amounts of recovered PET bottles, for example, have molecular weights substantially reduced by half. Therefore, using this as a base resin for reutilization will cause poor processability, where the original PET bottles cannot be made, but only monofilaments that can be formed with even low molecular weights may be formed. Thus, the current state of reutilization is limited to a narrow range applications.
As approaches for solving this problem, methods of restoring molecular weight by a solid phase polymerization of polyesters, of reacting a chain extender and polyester terminal groups, of adding other resins such as an elastomer to support mechanical characteristics, and the like are known.
As the chain extender, practical applications of compounds having functional groups, such as isocyanate, oxazoline, epoxy, aziridine, or carboimide have been proposed. However, practical compounds are limited because of strict restrictions in reactivity, heat resistance, stability and the like. Of these, epoxy compounds are relatively useful, and blends of monoepoxy compounds (Japanese Patent Laid-open No. Sho-57-161124), of diepoxy compounds (Japanese Patent Laid-open No. Hei-7-166419, Japanese Patent Publication No. Sho-48-25074, Japanese Patent Publication No. Sho-60-35944, and the like) are disclosed. However, there have been various problems in reaction rate, gel formation, melt viscosity, compatibility, heat stability, physical properties of the molded product, and the like.
On the other hand, a method of increasing the molecular weight of polyester by melting and mixing the recovered polyester with a bifunctional epoxy resin and a steric hindrance type hydroxyphenyl alkyl phosphorous acid ester hap been proposed (Japanese Patent Laid-open No. Hei-8-508776). Although this method may provide a relatively fast reaction rate, the steric hindrance type hydroxyphenyl alkyl phosphorous acid ester used is expensive, and there is a problem with practical use in the industry where low recovery and recycling costs are required. There have also been proposed methods of blending rubber or elastomers with the polyester, but in such case, there have been difficulties in compatibility, heat resistance, modulus of elasticity and the like.
An object of the present invention is therefore to provide a process for producing polyester resin pellets having increased molecular weight and molecular weight distribution and improved processability using a polyester resin having a relatively low molecular weight as a raw material, and a process for producing a molded product of the polyester resin.
As a result of extensive investigations to solve the above-mentioned problems, the present inventors have discovered that the above-mentioned object can be attained in an industrially advantageous fashion by blending a specific epoxy compound as a binder and a bond reaction catalyst with a saturated polyester followed by heat fusion to reach the completion of the present invention.
That is, the present invention provides, in one aspect thereof, a method for producing a high molecular weight polyester having swell of 1-200%, characterized by heating a mixture consisting of: (a) 100 parts by weight of a linear saturated polyester; (b) 0.3-10 parts by weight of a mixture, as a coupling agent, composed of (1) 0-100 percent by weight of a bifunctional epoxide ingredient containing 2 epoxide groups per molecule and (2) 100-0 percent by weight of a polyfunctional epoxide ingredient containing 3 and more epoxide groups per molecule; and (c) 0.01-5 parts by weight of metal salt of carboxylic acid as a catalyst for coupling reaction, at a temperature higher than a melting point of said polyester resin.
Further, the present invention provides the above mentioned production process, characterized in that metal salt is selected from a group consisting of a sodium salt and a manganese salt of carboxylic acid.
Also, the present invention provides the above mentioned production process, characterized in that the linear saturated polyester is a recycled molded product of polyester and having an inherent viscosity of 0.30-0.90 dl/g.
Still yet, the present invention provides the above mentioned production process, characterized in that the epoxide ingredient containing 2 epoxide groups per molecule contains at least one selected from a group consisting of aliphatic polyethyleneglycol diglycidylether, aromatic bisphenol A diglycidylether and its pre-condensate.
Also, the present invention provides the above mentioned production process, characterized in that the epoxide ingredient containing 3 epoxide groups per molecule contains at least one selected from a group consisting of aliphatic trimethylolpropane triglycidylether, an aromatic phenol novolac epoxy resin, a cresol novolac epoxy resin and bisresorcinoltetraglycidylether.
Additionally, the present invention provides a method of production of a high molecular weight polyester having swell of 1-200%, characterized by heating a mixture consisting of: (a) 100 parts by weight of a linear saturated polyester; (b) 0.3-10 parts by weight of a mixture, as a coupling agent, composed of (1) 0-100 percent by weight of a bifunctional epoxide ingredient containing 2 epoxide groups per molecule and (2) 100-0 percent by weight of a polyfunctional epoxide ingredient containing 3 and more epoxide groups per molecule; and (c) at least one coupling reaction catalyst selected from a group consisting of (1) carboxylate, carbonate, bicarbonate of alkali metal, (2) carboxylate of alkaline earth metal, (3) carboxylate of aluminium, zinc or manganese, (4) manganese carbonate, at a temperature higher than a melting point of said polyester resin.
Further, the present invention provides a polyester resin produced according to the above mentioned production process.
Also, the present invention provides a method for producing molded articles such as injection molding products, thin films, sheets, yarns, bottles and fibers by using the above mentioned polyester.
Further, the present invention provides a method for producing molded articles such as injection molded products, films, yarns, bottles and fibers comprising: heating a mixture of (a) 100 parts by weight of a linear saturated polyester, (b) 0.3-10 parts by weight of a mixture, as a coupling agent, consisting of (1) 0-100 percent by weight of a bifunctional epoxide ingredient contains 2 epoxide groups per molecule and (2) 100-0 percent by weight of a polyfunctional epoxide ingredient contains 3 and more epoxide groups per molecule, and (c) 0.01-5 parts by weight of metal salt of carboxylic acid as a catalyst for coupling reaction, at a temperature higher than a melting point of said polyester resin.
Additionally, the present invention provides the above mentioned production process for producing molded articles, characterized in that the linear saturated polyester is flakes of recycled PET bottles.